1. Field of the Invention
The present invention relates to a laser irradiation method for irradiating laser beams and to a laser irradiation apparatus (an apparatus including a laser and an optical system that leads laser beams produced from the laser to an irradiation object) for implementing the irradiation method. Further, the present invention relates to a manufacturing method for a semiconductor device that is manufactured according to steps including irradiation of a laser beam. The semiconductor device mentioned above refers to an electrooptical device such as a liquid crystal display device or a light-emitting device and to an electronic device including the electrooptical device as a component.
2. Description of the Related Art
Techniques having widely been researched in recent years include those for producing a crystalline semiconductor film in such a manner that a semiconductor film formed on an insulating substrate formed of, for example, a glass substrate is annealed with laser beam light and is crystallized to improve crystallinity. Throughout the present specification, the crystalline semiconductor film refers either to a semiconductor film including a crystallized area or to a semiconductor film crystallized overall.
Compared with a synthetic quartz glass substrate, a glass substrate is advantageous in that a large area substrate can easily be produced at low costs. The laser is frequently used in crystallization for the glass substrate, because the glass substrate has a low melting point. The laser is advantageous in that it imparts a high energy to a semiconductor film without greatly increasing the temperature. Further, a significantly high throughput can be obtained in comparison to the case of a heat treatment performed with an electrothermal oven.
A crystalline semiconductor film formed with laser beam irradiation exhibits a high mobility. Thus, the semiconductor film is used for, for example, an active-matrix liquid crystal display device. The active-matrix liquid crystal display device is manufactured such that thin film transistors (TFTs) are formed using crystalline semiconductor films on a single glass substrate so that the TFTs are used for either pixel portions or pixel portions with a driver circuit.
Laser annealing is performed in a method in which a high-power pulse laser beam generated by an excimer laser or the like is processed by an optical system to be shaped as, for example, a several centimeter square spot, a 10 centimeter or longer line, or a rectangle, on an irradiation surface. Concurrently, the laser beam is scanned (alternatively, an irradiation position of the laser beam is moved relative to the irradiation surface). Since this method exhibits high productivity and provides industrial advantages, the method is preferentially employed.
In particular, according to the method of using the spot-like laser beam, scanning needs to be performed backward-forward and rightward-leftward on an irradiation surface. However, according to the method of using the linear laser beam, the laser beam can be irradiated to the entirety of the irradiation subject surface only by performing scanning in a direction perpendicular to the linear direction of the linear beam. The scanning is thus performed in the direction perpendicular to the linear direction for the reason that the direction is the most effective in terms of productivity. As a current mainstream technology, laser annealing is performed using the method in which a laser beam emitted from an excimer laser of a pulse-oscillation type is processed using an appropriate optical system, and a beam thereby shaped linear on an irradiation surface is used for irradiation.
As an example, FIGS. 6A and 6B show a configuration of an optical system disclosed in JP 2001-244213 A. The optical system is used to process a laser beam on an irradiation surface and to thereby shape the beam linear. First, the configuration will be described with reference to a side view in FIGS. 6A and 6B. A laser beam passed through a laser oscillator 101 travels straightforward with a divergence angle. Subsequently, the laser beam is processed into collimated beams through cylindrical lenses 104 and 105, and split beams then converge on an irradiation surface 107 via a mirror 106. Since the optical system shown in FIGS. 6A and 6B are easily influenced by variations in the divergence angle, the system needs to be controlled. Thus, the configuration preferably uses an optical system that produces collimated beams that are not influenced by the divergence angle. Although a completely collimated beam cannot be produced, divergence of the beam can be minimized. An optical system of the type described above is called a xe2x80x9cbeam collimatorxe2x80x9d. In the configuration shown in FIGS. 6A and 6B, the cylindrical lenses 104 and 105 serve as collimators. The mirror 106 is shaped to have a plurality of parabolic mirrors each having a different curvature (mirrors each having only a unidirectional curvature), on which the beams converge at the focal points and are then led to reach the irradiation surface 107. In the example, four parabolic mirrors are provided. Since the curvatures of parabolic surfaces are different from one another, the focal points are also different from one another. According to parabolic mirrors 106a to 106d, energies of linear beams are homogenized in the beam cross direction, and the beam lengths thereof are determined.
Next, the configuration will be described with reference to a top view in FIG. 6B. A laser beam passed through the laser oscillator 101 is split through a cylindrical array lens 102 in a direction perpendicular to the traveling direction of the laser beam light. In the present specification, the aforementioned direction is referred to as a xe2x80x9ctransverse directionxe2x80x9d. With a mirror inserted in the course of the optical system, the transverse direction is variable to a direction of light bent by the mirror. In this configuration, the laser beam is split into four laser beams. The split laser beams converge through a cylindrical lens 103 into a single laser beam on the irradiation surface 107.
Generally, as a lens for transmitting laser beams is repeatedly used, the lens gradually deteriorates and becomes unusable. However, dissimilar to such a lens, a mirror does not transmit laser beams but reflects the laser beams impinged against its surface. That is, only the surface of the mirror is deteriorated. Thus, even in a case where the mirror is used for a long time to an extent of causing the surface thereof to deteriorate, the mirror can be rectified to be usable by recoating the deteriorated surface. From this viewpoint, the mirror is economically advantageous.
However, an energy density distribution of a laser beam formed on the irradiation surface 107 according to the optical system shown in FIGS. 6A and 6B are as shown in FIG. 7. Such a distribution is attributed to continual variations in the energy density. The variations are caused due to lens aberrations in the individual mirrors and differences in optical lengths to the irradiation surface. Since inhomogeneous energy density distributions of the aforementioned type with individual mirrors are synthesized on the irradiation surface 107, inhomogeneity is increased significant.
The energy density distribution of the beam formed on the irradiation surface 107 is preferably homogeneous to perform homogeneous laser annealing for an irradiation object. For example, when the energy density is homogeneous in processing a semiconductor film used as an irradiation object, the semiconductor film can be annealed homogeneous, thereby having homogeneous physical property. Consequently, a TFT manufactured using the semiconductor film thus annealed reduces the inhomogeneity in electrical characteristics. Consequently, improvement can be attained in operational characteristics and reliability of a semiconductor device produced using TFTs manufactured in the manner described above.
Accordingly, an object of the present invention is to provide a laser irradiation apparatus for forming a laser beam exhibiting a homogeneous energy density distribution on an irradiation surface. Another object of the present invention is to provide a laser irradiation method using the laser irradiation apparatus. Still another object of the present invention is to provide a method of manufacturing a semiconductor device by using a semiconductor film obtained by performing crystallization of the semiconductor film and activation of an impurity element according to the laser irradiation method.
The present invention causes laser beams exhibiting energy density distributions different in inhomogeneity on an irradiation surface to be overlapped, and thereby forms a laser beam exhibiting a homogeneous energy density distribution.
A laser irradiation apparatus disclosed in the present specification has a configuration including a laser and at least two mirrors each having a concave surface for unidirectionally homogenizing an energy density of laser light emitted from the laser. A focal position of a first mirror exists between the first mirror and an irradiation surface. A focal position of a second mirror does not exist between the second mirror and the irradiation surface, but exists behind the irradiation surface.
In the above-described configuration, a laser is one of solid-state, gas, and metal lasers of a continuous oscillation or pulse oscillation type. Examples of the solid-state laser include a YAG laser, an YVO4 laser, an YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandrite laser, and a Ti:sapphire laser of a continuous oscillations or pulse oscillation type. Examples of the gas laser include an excimer laser, an Ar laser, a Kr laser, and a CO2 laser of a continuous oscillations or pulse oscillation type. Examples of the metal laser include a helium-cadmium laser, a copper vapor laser, and a gold vapor laser of a continuous oscillations or pulse oscillation type.
In the above-described configuration, the laser beam is preferably modulated into a harmonic by using a nonlinear optical device. For example, a YAG laser is known to produce a laser beam having a wavelength of 1,064 nm as a fundamental. The absorption coefficient of the source laser beam for a silicon film is very low, thereby making it difficult to crystallize an amorphous silicon film, which is a semiconductor film. However, using the nonlinear optical device enables the laser beam to be modulated into a shorter wavelength. In this case, as a harmonic, a second harmonic (532 nm) or a third harmonic (355 nm) is preferable. Since either of the harmonics has a high absorption coefficient for an amorphous silicon film, the harmonic can suitably be used for crystallization of the amorphous silicon film.
According to a configuration of a laser irradiation method of the present invention disclosed in the present specification, first and second mirrors individually having concave surfaces are used to irradiate the irradiation object with a laser beam exhibiting an energy density distribution unidirectionally homogenized either on the irradiation object or in the vicinity thereof. A focal position of a first mirror exists between the first mirror and an irradiation surface. A focal position of the second mirror does not exist between the second mirror and the irradiation surface, but exists behind the irradiation surface.
According to another configuration of a laser irradiation method of the present invention disclosed in the present specification, first and second mirrors individually having concave surfaces are used to split a first laser beam into second and third laser beams. The second laser beam converges through the first mirror, and an irradiation object is then irradiated. On the other hand, however, the third laser beam does not converge through the second mirror, and the same area of the irradiation object is irradiated with the laser beam.
In the above-described configuration, a laser is emitted from one of solid-state, gas, and metal lasers of a continuous oscillation or pulse oscillation type. Also, the laser beam is preferably modulated into a harmonic by using a nonlinear optical device.
According to a configuration of a method of manufacturing a semiconductor device disclosed in the present specification, first and second mirrors individually having concave surfaces are used to irradiate the semiconductor film with a laser beam exhibiting an energy density distribution unidirectionally homogenized either on the semiconductor film or in the vicinity thereof. A focal position of a first mirror exists between the first mirror and the semiconductor film. A focal position of the second mirror does not exist between the second mirror and the semiconductor film, but exists behind the semiconductor film.
According to another configuration of a method of manufacturing a semiconductor device of the present invention disclosed in the present specification, first and second mirrors individually having concave surfaces are used to split a first laser beam into second and third laser beams. The second laser beam converges through the first mirror, and the semiconductor film is then irradiated. On the other hand, however, the third laser beam does not converge through the second mirror, and the same area of the semiconductor film is irradiated with the laser beam.
In the above-described configuration, a laser beam is emitted from one of solid-state, gas, and metal lasers of a continuous oscillation or pulse oscillation type. Also, the laser beam is preferably modulated into a harmonic by using a nonlinear optical device.
In the above-described configuration, a substrate for forming the semiconductor film may be formed using one of, for example, a glass substrate, a quartz substrate, a plastics substrate, a metal substrate, and a flexible substrate. Examples of the glass substrate include those made of glass, such as barium borosilicate glass and alumino borosilicate glass. The flexible substrate refers to, for example, a film-state substrate made of PET, PES, PEN, acrylic resin, or the like. When a semiconductor device is manufactured using such a flexible substrate, the device can be expected to be lightweight. In this case, a barrier film formed of, for example, an aluminum film (AlON, AlN, AlO, or the like), a carbon film (DLC (diamond-like carbon) film or the like), or a SiN film, may either be mono-layered or be multi-layered either on an obverse surface or on obverse and reverse surfaces of the flexible substrate. This is preferable in terms of improving properties such as durability.